FOCUS on...Visual Prostheses (Part 2)

In Part 1, we looked into how vision is achieved and how damage to the retina by two common diseases impairs one's vision. While there aren't any cures for such diseases, implants are being developed to help those with age-related macular degeneration (AMD) and retinitis pigmentosa (RP) live easier lives.

One of the most well-known and well-developed projects comes from the Department of Ophthalmology and the Department of Biomedical Engineering at my alma mater, the University of Southern California. Led by Dr. Mark Humayun and Dr. James Weiland, with teams from USC, Caltech, UC Santa Cruz, and the National Science Foundation, with Second Sight Medical Products, Inc., the 10-year old project consists of a glasses-mounted camera, an implanted wireless receiver, and a small electrode array attached to the back of the retina. The camera captures an image and converts it into an electric signal, which is sent to the receiver implanted inside the eye. The receiver sends the signal to the electrode array, which emits electrical pulses perceived by the user as small points of light.

Current research includes developing an intraocular camera which would be placed into the space that the (now-useless) lens occupies. Users of the current glasses-mounted camera must move their heads to scan an object. An intraocular camera will allow users to move their eyes to scan an object in a more natural manner. Another ongoing project is increasing the number of electrodes on the array. The first model (Argus I) had 16 electrodes, allowing the user to basically see only a 4x4 field of pixels. The latest model has 60 electrodes, allowing the user to see much greater detail.

Other notable approaches include the Implantable Miniature Telescope (IMT), developed by Dr. Isaac Lipshitz with VisionCare. The IMT takes one's central field of vision and enlarges it up to 3x. In this way, the image is not focused on the damaged part of the retina but is projected over other healthy parts of the retina, which reduces the blind spot that those with AMD suffer from.

Another unique approach is the Artifical Silicon Retina, developed by Drs. Alan and Vincent Chow. The ASR is implanted in the sub-retinal space in the eye and the unique composition of the device converts light into electrical current without the need of any external devices, basically mimicking the function of photoreceptors. However, it is shown that the device is not theoretically practical for neural stimulation. Moreover, Optobionics, the company that developed the ASR, ceased operations in May 2007.

Visual prostheses are exciting devices with numerous engineering challenges to overcome. The physiology of the eye only allows for an implant no larger than 5mm. The curvature of the eye presents challenges in developing electrode arrays with equal retinal stimulation. And as with almost any implant, there are issues of power and telemetry requirements. However, there is so much devoted to researching visual prostheses that we will undoubtedly have some amazing "bionic eyes" in the near future.

Learn More:

Wikipedia entry on visual prostheses

U.S. Department of Energy webpage on the Artificial Retina Project (USC project)

FOCUS on...Visual Prostheses (Part 1)

"FOCUS on..." will be a periodic DeviceLine feature in which a specific part of the body or disease will be highlighted. Basic anatomy and physiology or pathology will be covered, followed by current research and companies that are developing devices."

We've come a long ways in helping visually-impaired people to see. Maybe not as far as Star Trek's Geordi LaForge, who even in early part of his career had far superior vision than most of his crewmates even with his VISOR, but technological advancements have given blind people opportunities for greater independence. In the near future, Braille could very well become less common as prostheses are being developed to allow people to see the real world around them.

Overview of Vision: Vision is accomplished by a coordination of motor and sensory systems in the eye. Ocular muscles move your eye to the desired target. Light reflected off this target enters the eye. The amount of light that enters is adjusted by the pupil, which changes in shape to allow more or less light to enter. The cornea refracts the light, while the lens also refracts the light and focuses the light more precisely through the eye. The image is then projected in an inverted fashion onto the retina, which contain specialized cells called photoreceptors. These photoreceptors are responsible for changing light into electrical signals that are sent to the brain.

Diseases of the Eye: Patients who use visual prostheses are usually diagnosed with age-related macular degeneration (AMD) or retinitis pigmentosa (RP). Both are related to damage to the retina.

AMD is the leading cause of blindness in western countries and is typically caused by abnormal deposits called drusen between the retina and the blood vessels in the back of the eye. The result is a slow, increasing loss of one's center of vision. Those with AMD never go totally blind, however, their perception is greatly impaired, as shown in the photo on the right. There is currently no cure, and laser treatment is the only current option available.

RP is a genetic eye condition that is the leading cause of inherited blindness. RP results in slow degradation of the photoreceptors in the retina, and can lead to total blindness. Those who suffer from RP typically begin to lose their peripheral vision, giving them tunnel vision that gets smaller and smaller over time (shown on left). There's currently no cure for RP.

While both AMD and RP currently don't have any cures, there are treatments being developed in the form of implantable prosthetic devices.

Tomorrow in Part 2, we'll look into some of the devices being developed for treatment of AMD and RP and the companies behind them, as well as engineering challenges for future devices.